1. Field of Invention
The invention relates to a capacitive sensor with at least one reference impedance and at least one measuring condenser, with at least one electrical alternating signal source, with a current supply network as well as with an analysis unit, whereby the reference impedance and the measuring condenser are connected via the current supply network to the alternating signal source and the analysis unit in such a way that the charge and discharge currents of the reference impedance and the measuring condenser can be analyzed—at least partially—by the analysis unit. In particular, in this case, this can be a capacitive sensor with at least a first connection and a second connection, with at least one reference impedance and a measuring condenser, with at least one electrical alternating signal source, with an analysis unit as well as with a diode ring that consists of at least four diodes that are connected one behind the other in series and in the same direction, whereby the diode ring has a first connection between the first diode and the second diode, a second connection between the second diode and the third diode, a third connection between the third diode and the fourth diode, and a fourth connection between the fourth diode and the first diode, whereby the first electrode of the reference impedance is connected to the fourth connection of the diode ring, the second electrode of the reference impedance is connected to a reference potential, and the first electrode of the measuring condenser is connected to the second connection of the diode ring, whereby the first connection of the sensor is connected to the first connection of the diode ring, and the second connection of the sensor is connected to the third connection of the diode ring, whereby the first connection of the sensor and the second connection of the sensor are connected or can be connected to the analysis unit and whereby the electrical alternating signal from the electrical alternating signal source strikes the first connection of the diode ring and the third connection of the diode ring.
2. Description of Related Art
Capacitive sensors of the previously described type are known from the U.S. Pat. Nos. 5,650,730 and 5,793,217 and are used to determine the capacitance of the measuring condenser or the change in capacitance of the measuring condenser. In this case, in the capacitive sensor itself, often, only one electrode of the measuring condenser is designed, and the other electrode of the measuring condenser is formed by the surrounding area of the capacitive sensor. Thus, the measuring condenser is normally not a condenser in terms of a complete electrotechnical component, but rather an arrangement that is equipped with a capacitance whose active electrode is assigned to the capacitive sensor, whereby an electrical stray field extends from the active electrode into the surrounding area.
In the prior art, in most cases, a reference condenser is used as a reference impedance. Where there is concrete mention below of a reference capacitance, the embodiments still generally apply to a reference impedance; the capacitance of the reference condenser then corresponds to the value of the reference impedance, independently thereof, as the reference impedance is implemented as a component. The charge and discharge currents of a reference condenser then correspond to the charge and discharge currents of a reference impedance, whereby the reference impedance can convert the energy supplied to it in some other way than only in the electric field of a condenser.
The capacitance of the above-described sensor can be changed, on the one hand, if the geometry of the arrangement, and thus, the stray field of the active electrodes is changed; on the other hand, the capacitance of the sensor—without a change in the extension of the stray field—can also change in an alteration of the dielectric properties of the space, in which the electric field extends. Because of these general properties, capacitive sensors are frequently used as proximity switches and as fill-level detectors.
In capacitive sensors of the initially described type, the alternating signal source is usually designed as an oscillator, such as, for example, as a harmonic oscillator in the form of an LRC network, which is switched in such a way that it executes a continuous oscillation. As the signal level within the positive semioscillation of the alternating signal increases, the measuring condenser is charged via a current that flows over the first connection of the sensor, the first connection of the diode ring, and the second diode of the diode ring, and the reference impedance—frequently designed as a reference condenser—is charged during the latter with a current that flows via the second connection of the sensor, the third connection of the diode ring and the fourth diode of the diode ring. As the signal level of the positive semioscillation of the alternating signal decreases, the second diode and the fourth diode of the diode ring are blocked, via which the measuring condenser and the reference condenser had been pre-charged, while the previously locking first diode and third diode of the diode ring are now conductive. The charge stored during the charging process in the measuring condenser now flows over the third diode of the diode ring, the third connection of the diode ring, and the second connection of the sensor. During the negative semioscillation of the alternating signal, this process is repeated in a corresponding way.
The mode of operation of the above-described capacitive sensor is consequently based on the fact that the charge current of the reference impedance, which can be configured in particular as a reference condenser, or the charge current of the measuring condenser, in each case flows over a connection of the sensor that is different from the discharge current of the reference condenser or the measuring condenser. If the capacitances of the reference condenser and the measuring condenser are equally large, the current that flows into the first connection of the diode ring on average is equal to the current that flows out from the first connection of the diode ring, and the same applies for the third connection of the diode ring. If the capacitances of the reference condenser and the measuring condenser, however, vary in size, a resulting current that flows into the first connection of the diode ring and correspondingly flows out of the third connection of the diode ring is produced in the average time—for the case that the capacitance of the measuring condenser is larger than the capacitance of the reference condenser—and a current that flows out, in the average time, from the first connection of the diode ring and in a corresponding manner a current that flows, in the average time, into the third connection of the diode ring are produced—for the case that the capacitance of the measuring condenser is smaller than the capacitance of the reference condenser. By analyzing the differential currents with the analysis unit that is connected to the first connection and the second connection of the capacitive sensor, it is evident what the ratio is between the capacitance of the measuring condenser and the capacitance of the reference condenser.
In the generic capacitive sensors known from the U.S. Pat. Nos. 5,650,730 and 5,793,217, the currents that flow via the reference condenser and the measuring condenser into the analysis unit are fed via two current-voltage transformers to a summator, which processes the voltages with different signs so that a differential signal results. This differential signal is ultimately—after possible additional intermediate steps pertaining to circuit engineering—compared to a reference or threshold signal, whereby the reference signal defines a threshold, which, when reached, indicates a specific event, such as, e.g., a sufficient proximity of an object to the capacitive sensor or the presence/absence of a specific fill level.
The disadvantage to the above-described capacitive sensor is that the analysis of the current signals in the analysis unit is comparatively expensive. In particular, the specification of a reference value, to which the difference of the current voltage-converted currents is compared, is labor-intensive and costly in terms of circuit engineering, and in addition, is prone to frequency and amplitude fluctuations of the alternating signal. Also, it has been found that the known sensors, in the working frequency of 2 MHz indicated in the prior art, are not suitable to be used as fill-level sensors, since they then are not able to distinguish whether a medium fills a larger area of volume around the sensor or whether only a small adhesion of this medium to the sensor has been left after the medium has left the area of the capacitive sensor that is to be monitored; the fill level thus drops below the position of the capacitive sensor.